Silicone nitride is known to be very hard material which is suitable for manufacturing parts having high mechanical strength at high temperature (shafts, gas turbine blades, parts in contact with liquid metals, block bearings, ball bearings, sealing segments etc.), provided its degree of porosity is low. In this respect, the higher the degree of porosity of this material, the less it resists breakage forces and hot oxidation corrosion. A Si.sub.3 N.sub.4 of very low porosity can be manufactured which is suitable for the aforesaid applications by hot anisotropic mechanical pressing. By this method, compact Si.sub.3 N.sub.4 is obtained in the form of blocks, which are very costly to convert into mechanical parts because of the extreme hardness of the material, the special tools (diamond wheels) required for their machining, and the slowness of this work. Thus, an active attempt has been made during recent years to directly form parts by molding or stamping powder compositions based on Si.sub.3 N.sub.4, followed by hot sintering of the castings under an inert atmosphere. In doing this, the following three basic factors have proved important: the addition of densification aids, the use of powders of fine particle size (of the order of 1 to a few .mu.m) and, during sintering, the use of a relatively high nitrogen pressure, of the order of 2 to 50 atmospheres. By means of these improvements, densification levels of the order of 95 to 97% of the theoretical density are now expected (3.03-3.097 g/cm.sup.3).
The most important publications in this field include, for example: "Pressureless sintered silicon carbide" by I. ODA, M. KANENO and N. YAMAMOTO, Research and Development Laboratory, NGK Insulators Ltd., Mizuho, Nagoya, Japan; Yogyo Kyokai Shi 1976, 84 (8), 356-60 (Japan); Japan J. Mater. Sci 1976, 11 (6), 1103-7; Japanese Kokai patent specification 77 47,015; Yogyo Kyokai Shi 1977, 85 (8), 408-12; G. R. TERWILLIGER & F. F. LANGE, Journal of Materials Science 10 (1975), 1169-1174; U.S. Pat. No. 3,992,497 and "Sintering of silicon nitride" by D. J. ROWCLIFFE & P. J. JORGENSEN, Stanford Research Institute, Menlo Park, Calif.
The most used densification aids include MgO (5%); Al.sub.2 O.sub.3 +Y.sub.2 O.sub.3 (10-50%); BeO(1.25%)+MgO(3.75%); BeO(1.25%)+MgO(3.75%)+CeO.sub.2 (5%), etc.
It is nevertheless be desirable to attain even higher densification levels, in order to reduce the porosity levels, as the existing pores can be a source of cracks in the final pieces. To attain this, the theoretical density of 3.19 g/cm.sup.3 should be approached as closely as possible, while at the same time using as small a proportion of aids as possible in order to preserve the most favorable mechanical properties in the sintered material, such as hardness, resistance to bending, resistance to tensile stress and resistance to breakage, particularly at high temperature. Thus, it is useful to note that if powders are used containing relatively high proportions of densification aids, the mechanical properties, which are good at low temperature, can become mediocre at high temperature. Thus, if the aforesaid BeO+MgO+CeO.sub.2 is used, the modulus of rupture passes from 83 kg per mm.sup.2 at ambient temperature to 4 kg/mm.sup.2 at 1400.degree. C. On the other hand, Y.sub.2 O.sub.3 is expensive, and BeO is undesirable because of its toxicity. Moreover, it is economically preferable to sinter at atmospheric pressure rather than at a higher pressure, because in this manner the problems relative to the strength of materials at high temperature under pressure, such as sealing furnaces operating in the region of 1500.degree.-2000.degree. C., can be avoided.
Recently, pressureless sintering of a Si.sub.3 N.sub.4 powder containing 10 mol % of spinel (Mg aluminate) for 4 hours at 1850.degree. C. has been described, leading to a densification exceeding 96% and the formation of a material having an ultimate bending strength of 72 kg/mm.sup.2, which is high (see Yogyo Kyokai Shi 1976, 84 (10), 508-12). It is also interesting to note that according to this reference, it is not possible to obtain equivalent results by using MgO and Al.sub.2 O.sub.3 powders instead of spinel.